This invention relates to a positive type photosensitive anionic electrodeposition coating resin composition, a positive type photosensitive anionic electrodeposition coating material, an electrodeposition coating bath, an electrodeposition coating process and a process for producing printed circuit boards.
In production of printed circuit boards, first a layer of a photosetting resin composition is formed on a substrate, then the layer is irradiated imagewise with actinic light, and uncured portions are removed by development to form a resist pattern. In this process, various methods are employed for forming a layer of a photosetting resin composition. For example, there are known a method in which a solution of a photosetting resin composition (coating solution) is coated on a substrate by dip coating, roll coating, curtain coating or other coating techniques, and a method in which a film of a photosetting resin composition (photosensitive film) is laminated on a substrate. Among these methods, the method comprising laminating a photosensitive film is now predominantly used in the art as this method is capable of forming a photosetting resin composition layer with uniform thickness by a simple operation.
With a recent tendency toward higher density and higher precision of printed circuit boards, request is growing for the resist patterns of higher quality. That is, there are required resist patterns which are free of pin holes and have good adhesion to the underlying substrate surface. It is understood that the method comprising laminating a photosensitive film, which is now prevalently used, has a limit in meeting said request. According to this method, conformability to unevenness of substrate surface caused by scars at the time of substrate production, ununiformity of polishing, interstice of glass cloth forming an inner layer of the substrate, ununiformity of pits in copper plating on the surface, etc., is poor, and it is difficult to obtain sufficient adhesiveness. Such difficulty can be removed to some extent by carrying out the lamination of photosensitive film under reduced pressure (see Japanese Patent Examined Publication No. 59-3740), but a special and expensive apparatus is required for such practice.
Under these circumstances, interests have been shown again recently toward solution coating methods such as dip coating, roll coating, curtain coating, etc. These coating methods, however, have problems of difficulty in controlling film thickness, unsatisfactory uniformity of film thickness, formation of pin-holes, etc.
In the efforts for overcoming these problems, several proposals have been made on positive type photosensitive electrodeposition coating materials which are considered useful for producing printed circuit boards having through-holes. However, these coating materials, in most of which a quinonediazido group is used as photosensitive group, still had problems such as low photosensitivity. So, a new proposal has been made on an electrodeposition coating material containing a chemically amplified positive type photosensitive material comprising a combination of a compound which generates an acid upon irradiation with actinic light and a compound which has the nature of being decomposed by said generated acid and bettered in solubility in the developing solution, this coating material being far higher in photosensitivity than the conventional ones using a quinonediazido group (EP 0489560 Al).
These positive type photosensitive anionic electrodeposition coating materials, however, had a problem in that the copper ions eluted from the copper-clad laminate in the course of electrodeposition coating would form a chelate with the carboxyl group in the resist material to give rise to quasi-crosslinkage, so that when development is carried out with an alkali solution in a step after exposure, the exposed portion may not be developed sufficiently (hereinafter referred to as "underdevelopment").